1. Field of the Invention
The invention relates to the field of biochemistry. Provided herein are isolated laccase enzymes and nucleic acids encoding them. Also provided are mediators for laccase reactions. Also provided herein are methods for using laccases to oxidize lignins and other phenolic and aromatic compounds, such as for bio-bleaching and decolorization of wood pulp under high temperature and pH conditions to facilitate a substantial reduction in use of bleaching chemicals, as well as for treatment of fibers.
2. Description of the Related Art
Wood fiber is a multi-layered structure consisting primarily of cellulose, hemicellulose and lignin. Lignin is an insoluble complex polymer of phenolic compounds. Up to 90% of the lignin is solubilized and removed during the pulping process. The remaining lignin is a major cause of residual color in the pulp and must be removed by oxidative degradation or bleaching. The bleaching process requires application of harsh chemicals and conditions that are energy-intensive. Use of enzymes that assist in the bleaching process can allow reduced use of bleaching chemicals, increased energy efficiency of pulp plants, and have environmental benefits due to reduced chemical waste streams.
Filamentous fungi are able to efficiently degrade lignin by the action of several secreted enzyme classes. Of these, laccases have attracted considerable interest for application in pulp bio-bleaching. Laccases are multi-copper oxidases that couple direct oxidation of aromatic compounds with the reduction of molecular oxygen to water. During lignin degradation, laccases are thought to act on small phenolic lignin fragments that then react with the lignin polymer resulting in its degradation. Alternatively, artificial mediator compounds can be provided to accelerate the delignification process.
The wood pulping process can involve alkaline conditions. However, most known laccases are acidic enzymes. Only a few neutral or alkaline laccases have been reported; a laccase from Rhus vernificera has pHopt 9 and a laccase from Melanocarpus albomyces has a neutral pHopt on phenolic substrates. However, both of these laccases are only capable of oxidizing relatively low redox potential mediators that are not likely to oxidize lignin. Laccases are also likely to be exposed to relatively high temperatures in the pulp bio-bleaching application. Laccase enzymes that function well under the high temperature and pH conditions of typical pulping processes are desirable.